1. Field of the Invention
The present invention relates to an image stabilizing apparatus comprising a vibration detection means for detecting a vibration deviation amount with respect to an absolute space, an optical correction means for varying an optical path incident on a photographing lens with respect to the optical axis, and a control means for driving the optical correction means according to the output from the vibration detection means.
2. Related Background Art
In a conventional image stabilizing apparatus of this type, an angular deviation sensor shown in FIG. 19 is used.
In FIG. 19, in a case 301 in which a liquid 300 having a predetermined specific gravity is sealed, a float 302 having the same specific gravity as that of the liquid 300 is placed in a state wherein the float 302 is supported to be rotatable about a rotating shaft 303. The float 302 comprises a permanent magnet which is magnetized in a direction of an arrow a in FIG. 19, and forms a closed magnetic circuit with a yoke 304. A coil 305 is arranged between the float 302 and the yoke 304. When a current flows through the coil 305, since a force based on the Fleming's left-hand rule acts on the float 302, the float 302 is electrically controlled by the force.
In this state, if the case 301 which is moved integrally with a camera main body is rotated by ".theta..sub.IN " with respect to the absolute space by a camera shake or vibration, the float 302 maintains a static state with respect to the absolute space by the inertia of the liquid 300. For this reason, the float 302 is rotated by about ".theta..sup.IN " relative to the case 301. Therefore, the movement of the float 302 can be detected by an optical means which is moved integrally with the case 301 and includes an infrared light-emitting element (to be referred to as an iRED hereinafter) 306 and a semiconductor position detector (to be referred to as a PSD hereinafter) 307.
On the other hand, as a correction means for removing an image blur actually passing through a photographing lens on the basis of the output from the angular deviation sensor, a variable vertical angle prism sealed with a liquid 400 having a predetermined refractive index, as shown in FIG. 20A, is used.
In FIG. 20A, in the variable vertical angle prism, the internal transparent liquid 400 is sandwiched between two transparent plates 402, the outer surfaces are sealed by a resin film 403, and the entire structure is clamped by a frame body 404. The transparent plates 402 are respectively rotatable about rotating shafts 401a and 401b.
In FIG. 20A, light emitted from a point O on an object plane passes through the above-mentioned variable vertical angle prism and a photographing lens L, and forms an image on a film surface of a camera. If the camera is rotated by ".theta." with respect to the absolute space by a camera vibration of a photographer, and the point O on the object plane is relatively shifted to a position O', an image F on the film surface is moved to a position F'. In FIG. 20A, since the variable vertical angle prism is not moved at all, no correction is performed for the above-mentioned image blur.
In FIG. 20B, the transparent plate 402 near the photographing lens L of the variable vertical angle prism is inclined from a parallel position by an angle .delta. about the rotating shaft 401a. If the refractive index of the internal liquid 400 is represented by n, light from the point O' is deflected by ".theta.'" by the variable vertical angle prism so as to satisfy: EQU .theta.'=(n-1).delta.
Therefore, if ".theta.=.theta.'", an image is formed at the same image position as that free from any camera vibration.
Therefore, when the vibration deviation .theta..sub.IN of a photographer is detected by the angular deviation sensor, and feedback control is made so that the output from the sensor is always equal to the correction angle .theta. of the variable vertical angle prism in the optical axis direction, image stabilizing control can be performed without being influenced by a disturbance (e.g., a friction about the rotating shaft of the variable vertical angle prism).
However, in the conventional control method of the apparatus, feedback control must be executed while actually detecting the movement of the variable vertical angle prism so as to reduce the influence of, e.g., a disturbance. For this reason, the open frequency characteristics of the feedback loop considerably change due to a drift of a power supply voltage (which may change the loop gain depending on a method of, e.g., a driver), a change in temperature (which may change the viscosity of the liquid in the case of the variable vertical angle prism), and the like. In this state, the entire system may oscillate.
In the conventional apparatus, feedback control of the entire system is made so that the output value from the vibration detection means becomes equal to the output value indicating the moving amount of the correction optical system (optical correction means) provided to the photographing optical system. When the moving amount of the optical correction means is set in advance at a level necessary for correcting an actual camera vibration per unit angle, an image on the film surface passing through the photographing optical system can always be formed at the same point by the above-mentioned feedback control operation in an ideal state even when the camera main body vibrates. However, in practice, the output value per unit angle from the vibration detection means and the output value per unit correction angle from the optical correction means are not always constant due to variations of the mounting precision of a position detection sensor such as a PSD, the precision of a detection circuit, and the like. Thus, the stability of the image on the film surface is considerably impaired as compared to the ideal state.
Furthermore, according to the arrangement of the angular deviation sensor of the conventional apparatus, as the characteristics of the sensor are expanded to the lower frequency side, a force (a so-called spring force) for returning the float 302 (FIG. 19) to a reference position with respect to the case 301 must be weakened. In general, the liquid in the sensor has substantially the same specific gravity as that of the float 302. However, in practice, a specific gravity error is slightly included, and the float 302 itself is difficult to have a completely symmetrical shape due to its dimensional precision. For this reason, the float 302 is influenced by gravity. If a force actually acting on the float 302 by gravity is represented by mg, and a spring constant as a spring force for returning the sensor to the reference position is represented by K, an angular deviation given by ".theta.=mg/k" appears as a steady difference. When this value is extremely increased, light reflected by the float 302 to detect the position of the float 302 relative to the case 301 falls outside the PSD 307, and detection is undesirably disabled.